Preheat the oven to 225 degrees F. Line two baking sheets with parchment paper or silicone baking mats. Set aside.
In the bowl of a stand mixer fitted with a whisk attachment (or using an electric hand mixer), beat the egg whites at low speed until foamy. Add the cream of tartar. Continue to beat at low speed until soft peaks form.
Increase to medium speed while adding the sugar 1 to 2 tablespoons at a time, allowing the sugar to dissolve before adding more sugar. Once all the sugar has been added, increase to medium-high speed. Whip to glossy, stiff peaks, about 6 to 8 minutes. Add in the salt and vanilla extract. Mix until incorporated.
Fit a pastry bag with a large star tip (or desired piping tip). Fill the pastry bag with the meringue. Pipe 1 1/2-inch to 2-inch diameter cookie mounds about 2-inches apart on the prepared baking sheets. If desired, add sprinkles on top.
Bake for 1 hour. Turn off oven (don't open it). Let the meringue cookies remain in the oven for another 30 minutes. Slightly open the oven door and use a wooden spoon to keep the door ajar. Let the cookies rest in the oven for another 30 minutes, until they have cooled to room temperature.
Meringue cookies should be dry and crispy on the outside. Peel the meringue cookies from the parchment paper or silicone mt. Store at room temperature in an airtight container.
Room temperature egg whites will whip stronger and fluffier than cold egg whites.
Ensure mixing bowl and whisk (beaters) are clean. Any trace of fat (be it egg yolks, oil, or butter) will prevent the egg whites from forming stiff peaks.